62 MOUNTING AND PREPARATION OF OBJECTS. 



being regulated in the other tumblers to suit this, by 

 adding 6 and 7 to them and so on till we manage to 

 collect 12 single tumblers, whose contents represent 

 washings lasting from 5 to 60 minutes respectively, 

 this process will take a long time as the washings 

 multiply indefinitely; towards the end some of the 

 tumblers will be found to contain all the diatomaceas 

 tolerably free from impurities. The labour is not so 

 great as might appear at first sight, for every tum- 

 bler that has undergone twelve washings is put 

 aside. 



It has been recommended to mount diatomaceas 

 in bisulphide of carbon, as its high refractive index 

 makes their markings more distinct. We have as 

 yet had no experience of its efficacy in rendering 

 their details developable on a sensitised surface. 



Since the first discovery of the markings on the 

 diatomaceas their true nature has been the source of 

 endless strife, and is at the present moment in as 

 unsettled a condition as ever. I remember having 

 experienced immense delight many years ago, when 

 for the first time I perceived that the dots on some 

 of the coscinodesceas consisted of a coarse and fine 

 variety, and that I could reproduce exactly the same 

 appearance by placing two pieces of glass covered 

 with hemispherical elevations, within two or three 

 inches of each other. Since then my ardour with 

 regard to the investigation of the structure of 

 the diatomaceas has cooled, and I have made no 

 further attempt to prove how far the foregoing ex- 

 periment is an explanation of their appearance. My 



